Rhyolitic phreatomagmatism explored: Tepexitl tuff ring (Eastern Mexican Volcanic Belt)

Rhyolitic Tepexitl tuff ring in the Serdán-Oriental Basin of the eastern Trans-Mexican Volcanic Belt is a young, well-preserved circular crater - a simple morphology that belies a more complex eruptive history. Field observations, ash studies (granulometry, componentry, and morphology) and volatile data (LOI and XRF) allow the broad division of the Tepexitl deposits into two sequences: a lower sequence characterized by fine-grained deposits with abundant bomb and lapilli sags, and an upper sequence of coarse-grained deposits that show little to no deformation. The early eruptions at Tepexitl are interpreted to have been dominated by discrete, highly efficient, phreatomagmatic blasts, which caused a progressive deepening of the eruptive center (lower sequence), followed by a transition to dominantly magmatic behavior in the upper sequence. Dome growth occurred at the end of the eruption, but subsequent retrogressive explosions triggered by external water destroyed all trace of the original dome morphology. Molten fuel-coolant interaction (MFCI) is commonly thought to cause the repetitive water-magma interaction in phreatomagmatic eruptions, but a process by which magma and viscous felsic magmas can mingle prior to explosive interaction has not been described. A viable mechanism for rhyolitic MFCI, based upon field work at Tepexitl and laboratory experiments, requires that fluidized sediments intrude marginal fractures in the rhyolite magma, creating enough interfacial surface area to initiate phreatomagmatic explosions from within the interior of a rising plug or dome.